First of all, toponyms (like names of cities, rivers, mountains etc.), are not translated. If it is Hammranmyran in Norwegian, then it has to be transcribed (not translated) into Russian directly from Norwegian: "Хаммранмюран".
Secondly, what do you mean by "under" or "beneath"? Is that special place located at the bottom of the mire (under water and wet mud), or even lower underground? The translation may depen on that.

"We are going under Hammranmyran" can be translated as "Мы спустимся под Хаммранмюран", but it depends on the context a lot.